In these years, energy ray-curable resins typified by UV-curable resins have been used in various fields and applications, such resins being characterized in that only those portions thereof irradiated with energy ray at a certain dose or more are cured. On the other hand, energy ray typified by UV is characterized in that the energy ray is attenuated in the course of transmitting through the resin and hence hardly reaches the deep portion of the resin, or energy ray is largely attenuated or absorbed, for example, by those substances that absorb radiation having the same wavelength as that of the energy ray.
Accordingly, a light-curable resin suffers the following problems: a problem that a light-curable resin undergoes curing limited to a surface layer, reached by an energy ray, of a depth ranging from a few microns to a few millimeters, and thus, the deep portion of the resin remains uncured so as to be hardly applicable or inapplicable to thick-walled materials; and a problem that when a light-curable resin contains an energy ray transmission blocking substance such as a filler, curing failure is readily caused to result in incurability. Thus, the application ranges of such light-curable resins are mainly limited to photoresists, coatings, coating compositions, adhesives, varnishes and the like.
Typical examples of the solutions for such problems include: highly UV-curable resins (active energy ray-curable compositions, products of Mitsubishi Rayon Co., Ltd., Patent Document 1: Japanese Patent Laid-Open No. 8-283388); and UV/heat-curable resins (Optomer KS Series, products of Asahi Denka Ind. Co., Ltd.; Radicure, product of Hitachi Chemical Co., Ltd.; UE resins, products of Toyobo Co., Ltd., Patent Document 2: Japanese Patent Publication No. 61-38023). However, the highly UV-curable resins are still not free from a problem that such resins become incurable when energy ray is blocked by a filler or the like. The UV/heat-curable resins that undergo UV irradiation and subsequently heating are comparable in energy-ray curability level with conventional light-curable resins, and such problems involving thick-wall curing and curing with contained fillers have not been solved yet. Such problems are dealt with by heat curing (to be applied only to surface layer) due to heating conducted after light curing, and as affairs stand now, such problems have not been solved yet.
If a technique has been able to be established which can rapidly cure the above-described thick-wall resin containing an energy ray blocking substance and being large in attenuation and absorption of the energy ray, light-curable resins can be applied not only to the conventional application fields but to various other application fields to which light-curable resins have been hardly applicable because of the above-described problems possessed by light-curable resins; such various other application fields include an application to an FRP matrix resin, in particular, a CFRP matrix resin. For FRP, various processing methods and various production methods have hitherto been employed, most of the used matrix resins being heat-curable or thermoplastic resins. In molding of FRP, in particular, CFRP, the following problems are involved: the processing cost is high because the temperature control is complicated and the curing time is long; curing of a large size FRP requires a large size heating furnace; a resin curable in a short time at normal temperature cannot be used for a large size FRP that requires a long molding time; the temperature variation of the resin viscosity varies the resin impregnation condition to make the molding difficult; and the residual solvent generates voids at the time of curing of the resin to degrade the quality of the moldings.
Recently, application of light-curable resins to matrix resins has attracted attention as a solution for such problems as described above. Typical examples of such a matrix resin curing method may include, in particular, a filament winding molding method of Loctite Corporation which uses UV curing and heat curing in combination (Loctite Corp.; Fiber/resin composition and the preparation method thereof; Patent Document 3: National Publication of International Patent Application No. 1995-507836). However, in the FRP molding method using such a composition, a resin-impregnated and uncured FRP is subjected to UV radiation so as to cure the surface thereof and so as to extremely thicken (gelatinize) the interior thereof and thereby enable the maintenance of the shape and resin-impregnated state thereof to some extent, and is thereafter heated to perfect the curing.
Accordingly, although in the above-described molding method, as compared to conventional production methods based on a thermoplastic or heat-curable resin, the temperature variation of the resin viscosity is extremely small and the handling after impregnation is easy, the above-described molding method still involves the following unresolved problems: a heat curing step is required to perfect the curing, and hence the processing cost is raised due to heating and lighting cost and working hours required for heat curing; completion of the curing takes a long time; and molding of a large size FRP requires a large size heating furnace.
Accordingly, in view of the above-described drawbacks of the conventional energy ray-curable resin, FRP and particularly CFRP, the present inventors have a diligent study on the energy ray curing of the thick-wall resin containing an energy ray blocking substance and the energy ray curing of FRP, in particular, CFRP. Consequently, the present inventors have developed: a novel resin curing method that enables even energy ray curing of resin systems that contain highly energy ray-blocking substances such as carbon, carbon fiber (CF), metals and other inorganic fillers, wherein examples of such resin systems include carbon fiber reinforced composite materials (CFRP) and carbon/metallic substance/inorganic substance-containing resins; the compositions and molded products produced by the novel method; and a technique, as a molding method, related to chain-curing resin compositions (Patent Document 4: Japanese Patent Laid-Open No. 11-193322; Patent Document 5: Japanese Patent Laid-Open No. 2001-89639).
However, it has been revealed that the increase of the fiber volume content (Vf) of an FRP molded body formed even by using such a resin composition prevents the progress of the chain curing as the case may be.
Patent Document 1: Japanese Patent Laid-Open No. 8-283388
Patent Document 2: Japanese Patent Publication No. 61-38023
Patent Document 3: National Publication of International Patent Application No. 1995-507836
Patent Document 4: Japanese Patent Laid-Open No. 11-193322
Patent Document 5: Japanese Patent Laid-Open No. 2001-89639